mammal model for human antibody production

Most of the human proteins (eg. TNF-alpha) are produced also in other mammals. How can polyclonal anibodies be produced using mammals like goat, sheep, rabbits, mice when the orthologue protein introduced in them may not serve as an antigen?

The animal immune system won't produce antibodies to self proteins, so the antibodies produced in this case will be from portions of the protein that differ in some way from the human one. Because antibodies are so ridiculously specific, there is almost always a significant antigen portion present.